This invention relates to a high-frequency power oscillator comprising a multi-grid electron tube which includes at least a control grid, a screen grid and a cathode.
Such a high-frequency power oscillator is known from Dutch Patent Application No. NL 8600673 (U.S. Pat. No. 4,761,618). This known high-frequency power oscillator comprises a tetrode as an active element. When a tetrode is customarily used as an amplifying element in an oscillator circuit, the control grid is voltage controlled, while the control voltage may be superposed on a direct voltage for setting the control grid. The screen grid is then set to a fixed voltage value. Such a tetrode setting is also known from the above Dutch patent application.
One generally aims at achieving the highest possible power efficiency (i.e. the ratio between the a.c. power tapped from the anode and the d.c. power supplied to the anode) of an electron tube operating as an amplifier in a power oscillator. High efficiency is of major importance because the loss of energy is converted into heat. Since large powers are supplied and discharged with this type of electron tube, a considerably amount of heat in the anode of the electron tube will have to be dissipated in case of low efficiency. If this heat is not dissipated the electron tube will be destroyed.
In order to obtain maximum efficiency it is thus necessary for the a.c. voltage amplitude to have the same value as the d.c. voltage across the anode of this electron tube. This implies that the overall anode voltage (thus the sum of the a.c. and the d.c. voltages) can instantaneously become practically equal to zero volts. A known property of a multi-grid electron tube is the fact that when the anode voltage drops below the screen grid voltage, the anode current will decrease and the screen grid current will increase accordingly. Owing to this increase of the screen grid current the power dissipation in the screen grid will be enhanced, leading to a possible destruction of the screen grid.
In order not to cause a detrimental effect on the efficiency, it is desired to operate with a pulsating control of the control grid of the electron tube. When the electron tube is fully open or closed the heat dissipation will be low. In the former situation the heat dissipation will be low because the anode voltage is then momentarily zero and in the latter situation because the anode current is zero. Since both the anode voltage and the anode current are unequal to zero during the transitional period between the open and closed conditions of the electron tube and, consequently, much heat is dissipated, it is of major importance that the pulses have the steepest possible edges. Furthermore, steep edges enhance a proper definition of the pulse duration. A well-defined pulse duration is important for obtaining a balanced power control in cases of high pulse frequencies and different pulse durations.